1. Field of the Invention
The invention relates to a hydraulic control system for an automatic transmission. More specifically, it relates to a hydraulic control system which performs a clutch to clutch shift in which one frictional engagement element is engaged and an another frictional engagement element is disengaged at the same time to effect a shift to a predetermined gear ratio.
2. Description of the Related Art
In general, the different "vehicle driving states" include (1) a power on state in which an accelerator pedal is depressed during vehicle driving and a torque is transmitted from the engine to the vehicle wheels, that is to say, the torque is a positive value, (2) a power off state in which the accelerator pedal is released during vehicle driving and torque is not transmitted from the engine to the vehicle wheels but, rather, the torque is transmitted from the vehicle wheels to the engine, that is to say, the torque is a negative value, and (3) a state in which the vehicle changes from the power on state to the power off state. Automatic transmissions are designed to provide shift control in accordance with the particular vehicle driving states.
A conventional hydraulic control system for an automatic transmission is described in Japanese Kokai publication Hei 6-323415. In an up-shift by a clutch to clutch shift, the hydraulic control system detects the power on state or the power off state before the shift starts. Then, shift control is provided in accordance with one of plural predetermined control logics which correspond to the various vehicle driving states, based on the one detected vehicle driving state. In this prior art hydraulic control system, the output torque is detected by a sensor and then it is determined if the input torque of the transmission is a positive value or a negative value. When the input torque is a positive value, that is to say, when the vehicle driving state is the power on state, the shift is effected by controlling the engagement side pressure, and the disengagement side pressure is controlled so that a disengagement side frictional engagement element is disengaged at the end of a torque phase. When the input torque is a negative value, that is to say, when the vehicle driving state is the power off state, the shift is effected by controlling the disengagement side pressure, and the engagement side pressure is controlled so that the engagement side pressure is increased at the end of the shift. By these control schemes, shifts are performed smoothly without shock in the power on state and in the power off state, and a sensation of speed reduction during the power off state is avoided.
The conventional hydraulic control system requires a memory capacity for the hydraulic controls performed with different control logics for the power on state and for the power off state. The vehicle driving state is changed to the power on state during the shift in the power off state when, for example, a driver presses the accelerator pedal to maintain or increase the vehicle speed during an up-shift dictated by increasing vehicle speed with the vehicle powered by inertia or by driving on a down-hill where the throttle opening is low (e.g. 0). In such circumstances the disengagement side frictional engagement element is disengaged completely to avoid sensation of the speed reduction because the shift control is performed with the control logic determined at the shift start and the driving state at that time is the power off state. Therefore, when the driving state is changed to the power on state, engine racing occurs because no disengagement side pressure is applied to the disengagement side frictional engagement element.